Membrane filtration is a process used to separate a feed, or inlet, liquid into a product stream and a concentrate stream. Typically, the feed stream is water that needs to be filtered or desalinated so that it can be used for drinking, agricultural and industrial applications. In membrane filtration, a membrane acts as a barrier to allow certain compounds to pass through while rejecting others. One type of membrane filtration is reverse osmosis (RO) filtration, which is a pressure-driven process. During osmosis, water will diffuse from an area of high solute concentration to an area of low concentration due to osmotic pressure until an osmotic equilibrium is reached. Reverse osmosis is a process in which pressure is applied to a volume of high solute concentration in order to overcome the osmotic pressure and force the water in the high solute concentration to diffuse through the membrane to a low solute volume, thus leaving behind the solute. The membranes used in RO filtration are very selective and allow almost no solute to pass through.
One type of RO filtration system is known as a spiral wound element system. In this system, one or more membrane envelopes are wrapped around an elongate collection tube. Each membrane envelope comprises two membrane outer surfaces and a permeate sheet therebetween that communicates with holes in the sidewall of the collection tube. A feed spacer is disposed on one side of each membrane envelope such that when the membrane envelope is wrapped around the collection tube, a spiral configuration is formed with alternating layers of membrane envelope and feed spacer. The collection tube, membrane envelope(s), and feed spacer(s) combine to form a spiral wound element. Multiple elements are typically combined in series and parallel to process higher volumes of feed liquid.
In use, the spiral wound element is placed in a pressure vessel and water containing a high concentration of solute (known as feed water) is pumped, under pressure, into one end of the pressure vessel. The feed water enters the spiral wound membrane through the channels between the membrane envelopes created by the feed spacers and travels parallel to the axis of the collection tube. A portion of the feed water diffuses through the membrane and into the permeate sheet due to the high pressure of the feed water exceeding the osmotic pressure. The permeate sheet guides the water in a spiral direction until it reaches the collection tube and subsequently travels axially to the end of the spiral wound element. Feed water that does not diffuse through the membrane continues to travel in the axial direction and is typically transferred to another spiral wound element connected in series with the first spiral wound element.